Swapping in an ecoboost 3.5 would be a nitemare,
How so? The base engine for the F150 is the NA 3.7. The 6 spd. automatic tranny is standard in the F150...probably geared different though. But the truck tranny mates to both the 3.7 and the 3.5. If the Mustang's transmission is different from the truck's, it still bolts up to the 3.7 and might do the same on the 3.5.
If the 3.7 goes where the 3.5 goes, can't imagine the 3.5 Eco couldn't be made to fit in a Mustang...if you had the whole package.
Been ages since I did an engine swap though, so might be overlooking something. Educate me.